One of the more aggravating aspects of this kind of problem is, the computer is not able to provide much in the way of pointers or clues to the problem. Depending on one's experience level, there are various basic troubleshooting steps that may at least provide some clues. If you have a set of Recovery Disks (usually in the form of 3.5 " diskettes), try them and see if the PC will boot from them and at least come up in a more functional state. Generally (but not always) you will not have Recovery Disks if you did not make them yourself when you first got this computer.

Another option is to try booting in Safe Mode.

In recent years, there are somewhat more modern twists to the Recovery Disk option, such as a Bootable CD. If you have such a bootable CD and your Bios is setup to allow the CD drive as a boot source, you could try this too.

Even if you are able to do any of these, these won't necessarily fix anything, but if either works, this at least puts the computer into a better mode (compared to what you are experiencing now) from which to further troubleshoot. As it is now, system that hangs before completing booting, as you may well know, does not give you much at all to work with towards the effort of finding the problem.

Are you able to describe what, in general, had been done with this PC during the last time it was known to be working okay? Had it been showing any signs that perhaps it was ready to "go on the fritz" ? Had any changes been made? (new hardware added, old hardware removed, updated files, updated security patches, loading new programs, un-installing older programs, Registry changes, that kind of stuff?)

Good luck.